Light-cured Self-etch-adhesives Undergo Self-cure Initiated by Tertiary Amine and Hydroxyapatite

Objectives: To investigate effect of prior light-cure on model self-etch-adhesive’s ability to undergo self-cure mediated by ethyl 4-(dimethylamino)benzoate (4E) and hydroxyapatite (HAp). To elucidate if HAp released from dental substrate during etching process is adequate for self-cure phenomenon.
Methods: Bis[2-methacryloyloxy)ethyl] phosphate was mixed with 2-hydroxyethyl methacrylate at 1/1 (w/w) ratio, followed by deuterium oxide at 20wt%. Trimethylbenzoyl-diphenylphosphine oxide (TPO) was added at concentration of 0.022mmol/g monomer with or without 4E at 0.088mmol/g. 2-6% HAp was added to adhesives which were then cured at 550mW/cm² and fixed distance (30mm) for 10-20s. ATR-FTIR was used to monitor degree of conversion (DC) during light-irradiation and for additional 16h afterwards while stored in dark. Selected adhesives were injected into small incisions made in occlusal enamel of sound third molars. After 1minute’s etching, adhesives were light-cured for 10s at distance of 20mm, and specimens were stored in dark, water-saturated environment for 72h. Subsequent Raman line-mapping was performed across enamel-resin interface after specimens were fractured along notches created in advance.

Results: DC of 4E-free adhesives exhibited typical change over time, reaching ~25%, ~50% and ~75% after light-irradiation for 10s, 15s and 20s, and remained unchanged in subsequent 16h of dark storage. Adhesives with TPO and 4E reached DC similar to 4E-free counterparts upon completion of light-cure, but underwent spontaneous initiation which boosted DC to almost 100% regardless of HAp level or initial light-cure duration. When applied to enamel and cured, 4E-free adhesive had ~80% DC in resin which gradually descended to ~50% in enamel. In contrast, 4E-containing adhesive consistently scored ~80% DC across enamel-resin interface.

Conclusions: Inadequately light-cured model self-etch adhesive undergoes spontaneous polymerization initiated by 4E and HAp, salvaging resin curing. Such salvaging effect is present when adhesive is applied to dental substrate which releases HAp in etching process as initiator.